Skip to content

Format Specifiers 格式说明符

Here is a comprehensive reference guide for C language printf and scanf format specifiers, organized into detailed lookup tables.

1. Basic Conversion Specifiers

These are the core symbols used by both functions to define the data type.

Specifier Data Type printf (Output) Meaning scanf (Input) Meaning
%d int Signed decimal integer Reads a signed decimal integer
%i int Signed decimal integer Smart Read: Detects hex (0x), octal (0), or decimal
%u unsigned int Unsigned decimal integer Reads an unsigned decimal integer
%o unsigned int Unsigned octal (no prefix) Reads an octal number
%x / %X unsigned int Unsigned hex (lowercase/uppercase) Reads a hexadecimal number
%f float / double Decimal floating point (default 6 digits) Reads float only (Use %lf for double)
%e / %E float / double Scientific notation (e.g., 3.14e+02) Reads a floating point number
%g / %G float / double Shortest representation of %f or %e Reads a floating point number
%c char Single character Reads the next character (does NOT skip whitespace)
%s char * String (prints until \0) Reads string (stops at whitespace/newline)
%p void * Pointer address (implementation specific) Reads an address (usually hex)
%% None Prints a literal % Matches a literal % in input

2. printf Formatting Details

Prototype: %[flags][width][.precision][length]specifier

A. Flags (Control alignment, sign, padding)

Flag Description Example Result (Value: 5)
- Left Justify (Default is right) %-5d "5 "
+ Force Sign (Show + for positives) %+d "+5"
(space) Space for positive, - for negative % d " 5"
0 Zero Pad (Pad with 0 instead of spaces) %05d "00005"
# Alternate Form (0 for octal, 0x for hex) %#x "0x5"

B. Width & Precision

Format Description Example Result
%N Min Width N (pads with spaces) %5d (val 10) " 10"
%.N Precision (Decimal places for floats) %.2f (val 3.149) "3.15"
%.N Precision (Max characters for strings) %.3s (val "hello") "hel"
%.N Precision (Min digits for integers, pad 0) %.4d (val 10) "0010"
%* Dynamic Width (Provided by argument) printf("%*d", 5, 10) " 10"
*. ** Dynamic Precision (Provided by argument) printf("%.*f", 2, 3.1) "3.10"

3. scanf Input Control Details

Prototype: %[*][width][length]specifier

A. Input Modifiers

Feature Format Description Example
Width Limit %Ns Reads max N characters. Crucial for safety. scanf("%9s", buf) (Leaves room for \0)
Suppression %*d Reads input but discards it (does not assign). scanf("%d%*c%d", &a, &b) (Skips char between ints)
Scanset %[...] Reads only characters listed in brackets. scanf("%[A-Z]", str) (Only reads uppercase)
Negated Scanset %[^...] Reads until a character in brackets is found. scanf("%[^\n]", str) (Reads whole line until Enter)
Whitespace " " A space in the format string skips any amount of whitespace (spaces, tabs, newlines). scanf(" %c", &ch) (Skips previous newline)

4. Length Modifiers

Used to handle data types of specific sizes (short, long, size_t, etc.).

Modifier Integer (d, i, u, o, x) Float (f, e, g) C Type Mapping
hh signed/unsigned char - char (treated as integer)
h short - short
l (ell) long double (Required for scanf) long / double
ll (ell ell) long long - long long
L - long double long double
z size_t - size_t (e.g. result of sizeof)

5. Cheat Sheet & Examples

Common printf Scenarios

Goal Format String Value Output
Currency (2 decimals) "$%.2f" 12.5 "$12.50"
Zero-padded ID (5 digits) "%05d" 42 "00042"
Left-aligned column "|%-10s|" "Item" "|Item |"
Hex address (Standard) "%p" &val 00000064321A
Print size_t "%zu" sizeof(int) 4
Print long long "%lld" 9223372036854775807 9223372036854775807

Common scanf Scenarios

Goal Format String Notes
Read double "%lf" Must use l modifier. %f is only for float.
Read string with spaces "%[^\n]" Reads until newline character.
Read Date (YYYY-MM-DD) "%d-%d-%d" The - in the string matches and skips the dashes in input.
Skip specific prefix "ID:%d" If input is ID:500, it stores 500.
Safe String Read "%19s" If buffer is char buf[20]. Prevents overflow.

6. Important Notes

  1. Double Precision:
    • printf: %f works for both float and double (float is promoted to double).
    • scanf: You must distinguish: %f for float, %lf for double.
  2. Safety: Never use %s in scanf without a width limit (e.g., %100s). Unbounded %s is a major security vulnerability (buffer overflow).
  3. Return Values:
    • printf: Returns the number of characters printed.
    • scanf: Returns the number of items successfully read and assigned. Use this to check for EOF or bad input.
  4. Buffer Clearing: scanf("%c", &ch) reads the newline character left over from a previous scanf. Use scanf(" %c", &ch) (note the space) to skip it.